This invention relates to a gage for determining the location of the pitch cylinder of a threaded post or bore in a workpiece. The gage of the present invention is also useful in determining the amount of positional tolerance a threaded feature has used on the workpiece, orientation of the pitch axis relative to the surface of the workpiece, and thread feature tolerances.
Several methods exist for inspecting threaded elements on a workpiece, such as a threaded post or bore. Several features of the threaded elements are inspected, including position on the workpiece, orientation and concentricity with other features on the workpiece, perpendicularity with the surface of the workpiece, and pitch diameter. Typically, inspection of each of these features is performed with a separate gage.
For instance, positional accuracy is determined using a functional gage with hole locations corresponding to the correct bore locations in the workpiece. Separate unthreaded gage pins pass through the functional gage holes and into the workpiece bores only if the bore locations are within tolerance. In some instances, the unthreaded gage pins are integral with the functional gage. In either case, inspections of this type are not based on the pitch cylinder axis of the threaded element on the workpiece, which induces some error into the inspection. Moreover, even the use of a threaded gage pin does not necessarily establish the proper pitch cylinder axis due, for instance, to perpendicularity errors in the threaded workpiece bore.
The pitch cylinder is an imaginary cylinder formed such that the diameter of the cylinder intersects the thread profile, or flanks, where the width of the thread and groove are equal. Ideally, the pitch cylinder of a threaded feature is a cylinder formed by thread pitch diameters spaced along the length of the feature. The axis of the pitch cylinder can be defined using a minimum of four points of contact, axially and circumferentially spaced in two groups along the length of the threaded feature. Once the axis of the pitch cylinder is defined, positional aspects, for instance, can be easily gaged. To applicant's knowledge, there are no inspection gages available that accurately locate the axis of the pitch cylinder of the threaded feature to be inspected. A gage threaded to the pitch cylinder can establish the pitch cylinder axis only when it is exactly the size of the threaded feature, and when there is no play or slop when it is engaged onto the threaded feature. A threaded gage pin of this sort would be virtually impossible to engage, and, at any rate, relatively expensive to manufacture.
Typically, inspection gages are limited to a single function--i.e., determining positional accuracy, assessing perpendicularity, or inspecting the limits of size of the threaded feature. Thus, several gages are required to inspect one workpiece. Due to the precision machining of these gages, their cost is generally high. In order to reduce this cost, one option is to provide a multi-function gage, not yet available in the prior art, useful for inspecting thread position, orientation and limits of size of a threaded feature on a workpiece.